Add a "gn path" command for finding deps.

tools/gn/gn.gyp

 {
   'variables': {
     'chromium_code': 1,
   },
   'targets': [
     {
       'target_name': 'gn_lib',
       'type': 'static_library',
       'dependencies': [
         '../../base/base.gyp:base',
@@ skipping 16 matching lines @@
         'c_include_iterator.cc',
         'c_include_iterator.h',
         'command_args.cc',
         'command_check.cc',
         'command_clean.cc',
         'command_desc.cc',
         'command_format.cc',
         'command_gen.cc',
         'command_help.cc',
         'command_ls.cc',
+        'command_path.cc',
         'command_refs.cc',
         'commands.cc',
         'commands.h',
         'config.cc',
         'config.h',
         'config_values.cc',
         'config_values.h',
         'config_values_extractors.cc',
         'config_values_extractors.h',
         'config_values_generator.cc',
@@ skipping 219 matching lines @@
       'type': 'executable',
       'sources': [
         'generate_test_gn_data.cc',
       ],
       'dependencies': [
         '../../base/base.gyp:base',
       ],
     }
   ],
 }